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cpuidle: Fix last_residency division 

Snooze is a poll idle state in powernv and pseries platforms. Snooze
has a timeout so that if a CPU stays in snooze for more than target
residency of the next available idle state, then it would exit
thereby giving chance to the cpuidle governor to re-evaluate and
promote the CPU to a deeper idle state. Therefore whenever snooze
exits due to this timeout, its last_residency will be target_residency
of the next deeper state.

Commit e93e59ce5b "cpuidle: Replace ktime_get() with local_clock()"
changed the math around last_residency calculation. Specifically,
while converting last_residency value from nano- to microseconds, it
carries out right shift by 10. Because of that, in snooze timeout
exit scenarios last_residency calculated is roughly 2.3% less than
target_residency of the next available state. This pattern is picked
up by get_typical_interval() in the menu governor and therefore
expected_interval in menu_select() is frequently less than the
target_residency of any state other than snooze.

Due to this we are entering snooze at a higher rate, thereby
affecting the single thread performance.

Fix this by using more precise division via ktime_us_delta().

Fixes: e93e59ce5b "cpuidle: Replace ktime_get() with local_clock()"
Reported-by: Anton Blanchard <anton@samba.org>
Bisected-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Shreyas B. Prabhu 2016-07-01 09:24:14 -05:00 committed by Rafael J. Wysocki
parent 4c2e07c6a2
commit dbd1b8ea43
1 changed files with 4 additions and 8 deletions
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12
drivers/cpuidle/cpuidle.c
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@ -173,7 +173,7 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
struct cpuidle_state *target_state = &drv->states[index];
bool broadcast = !!(target_state->flags & CPUIDLE_FLAG_TIMER_STOP);
u64 time_start, time_end;
ktime_t time_start, time_end;
s64 diff;
/*
@ -195,13 +195,13 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
sched_idle_set_state(target_state);
trace_cpu_idle_rcuidle(index, dev->cpu);
time_start = local_clock();
time_start = ns_to_ktime(local_clock());
stop_critical_timings();
entered_state = target_state->enter(dev, drv, index);
start_critical_timings();
time_end = local_clock();
time_end = ns_to_ktime(local_clock());
trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
/* The cpu is no longer idle or about to enter idle. */
@ -217,11 +217,7 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
if (!cpuidle_state_is_coupled(drv, index))
local_irq_enable();
/*
* local_clock() returns the time in nanosecond, let's shift
* by 10 (divide by 1024) to have microsecond based time.
*/
diff = (time_end - time_start) >> 10;
diff = ktime_us_delta(time_end, time_start);
if (diff > INT_MAX)
diff = INT_MAX;